This invention relates to power transmission couplings, and particularly to couplings which utilize shear pins as a torque transmitting connection between coupling elements associated with the coupled shafts.
In certain applications of power transmission couplings connecting driving and driven shafts, such as in steel rolling mills and conveyors, the driven apparatus is likely to be jammed or stopped suddenly with a resulting overload which can damage the coupling and other elements in the power train. To prevent uncontrolled damage to the power train, the couplings are often provided with shear elements which form the weakest link in the drive train. These shear elements are designed to fail when a preselected overload is applied to the coupling so that sudden stoppage of the driven equipment or other causes of extreme shockloads will have the effect of breaking the shear element before failure of the coupling or damage to other elements in the drive train can occur.
The shear element is typically in the form of a replaceable pin. Examples of couplings employing shear pins are numerous in the prior art and include U.S. Pat. No. 1,978,209 issued Oct. 23, 1934, to Kuhns; U.S. Pat. No. 3,246,483 issued Apr. 19, 1966, to Schmitter; U.S. Pat. No. 3,855,818 issued Dec. 24, 1974, to Hochreuter; and British Patent No. 381,403 dated Oct. 6, 1932. The shear pins typically include a centrally located annular groove which defines the point of minimum cross section at which the pin should fail if it is subjected to pure shear loading.
In order to function as designed as to be subjected to shear loading only, the shear pin must fit without any clearance within the two coupling elements which it joins. Anything short of an interference fit will result in the application of bending stresses on the shear pin and this can lead to bending fatigue failures of the pins even when the overload has not been experienced.
However, it is very difficult to assemble a coupling with shear pins with an interference fit. As a result, the shear pins are typically assembled with some clearance and bending stresses necessarily result. Because of this, it has become accepted practice to replace the shear pins on a scheduled basis even though no failure has occurred but instead as a precaution against the possibility of fatigue failure. This results in unnecessary down-time for the equipment being driven.
Following failure of the shear pins it is often difficult to remove both ends from their respective bores in the couplings elements unless access can be gained from each end of the pins. Even if access is provided from each end, it is often difficult to replace an unbroken pin because the two coupling elements will typically have moved angularly a small amount thereby offsetting slightly the axes of the respective bores. To reduce such problem it has been common to utilize so-called stepped shear pins in which the diameter on one end of the pin is smaller than the diameter on the other end, and the diameters of the respective bores in which the pin ends fit are also of different sizes. This allows withdrawal of the smaller end through the larger diameter bore. Examples of the stepped shear pins are found in U.S. Pat. No. 3,855,818 and British Patent No. 381,403. The use of stepped shear pins increases the expense, however, because they are more difficult to machine than a pin of constant diameter, and they also require different sized bores or bushings therefore increasing the number of different parts required for the couplings.
By the present invention, I have provided a shear pin coupling which permits ease of assembly and replacement of the shear pins while at the same time providing a fit which approaches that of an interference fit thereby significantly reducing bending moments on the pin.